A telephone system does five separate jobs: it turns a voice into a varying electric current, carries that current over a pair of wires, signals that someone wants to talk, connects the right two pairs together, and powers and protects the line. Two instruments and one pair of wire need only the first three; every instrument added beyond that is what forces switching, an exchange and a power plant. The voice band it must carry is only about 300 to 3,400 Hz.
Part of Telecommunications
The parts of a voice network from instrument to exchange to long line, and which problem each part exists to solve.
Two Phones And One Pair Of Wire Need Only Three Of The Five Jobs
The smallest working telephone system is two instruments joined by a pair of wires. The transmitter (microphone) turns the pressure waves of speech into a varying current, the pair carries that current to the far end, and the receiver (earpiece) turns it back into sound. A hand-cranked magneto and a bell supply the third job, signalling, so that the other party knows to pick up. Nothing else is required, which is why the carrier article can describe a working telephone as little more than a carbon microphone, an electromagnetic earpiece, a battery and wire.
Each of those three jobs has its own article here. How the voice becomes current is in Microphone, and how current becomes sound again is in Speaker. The complete circuit, with its sidetone control and ringer, is already written up in Telephone Circuit, and ringing and dialling in Signaling and Ringing.
A Voice Needs Only 300 To 3,400 Hz Carried Faithfully
Human speech occupies roughly 300 to 3,400 Hz for telephone purposes, and the band is chosen because it keeps the vowel-defining resonances and enough consonant energy for words to be recognised (see Sound Wave Basics). Everything in the system is sized to that band: the diaphragms, the transformers, the filters, and later the width of each channel on a shared line. A telephone does not need to reproduce music, and it is cheaper and clearer for not trying.
The Fourth Job, Connecting, Is What Makes It A System
Connecting every pair of instruments directly takes N × (N − 1) / 2 pairs of wire for N instruments:
- 10 instruments: 10 × 9 / 2 = 45 pairs
- 100 instruments: 100 × 99 / 2 = 4,950 pairs
An exchange replaces that with one line from each instrument to a central point, so 100 instruments need 100 lines. The exchange is the first thing in the system that is more than wire, and it is the subject of Switching Networks. Its first version is a person, which is why Manual Exchange is as much about trust as about plugs.
The Fifth Job Is Power And Protection
A carbon microphone does not generate a signal. It controls current supplied by a battery, so some battery must be in the loop. A local battery in every instrument is simple but means visiting every house to maintain it; a single central battery at the exchange, commonly 24 V or 48 V, feeds every instrument through its own line. At 48 V through a loop of 1,000 Ω the current is 48 V / 1,000 Ω = 0.048 A, or 48 mA, inside the 15 to 100 mA that Battery and Powering gives for a carbon microphone.
Protection means lightning and mistakes. A kilometres-long wire is an antenna, and Lightning Protection puts the induced voltage from a nearby strike at thousands of volts.
What You Need
- A pair of insulated copper conductors between every instrument and the exchange; the choice of wire is in Wire Selection for Telephony
- A transmitter and a receiver at every instrument, both buildable from salvage
- A ringing arrangement, either a magneto and bell or an exchange ringing generator
- A power source, battery per instrument or one central battery
- A person or a machine at the centre once there are more than a few instruments
Common Mistakes
- Treating the instrument as the whole system. A pair of phones works on a bench and fails when a third is added, because nothing connects, signals or isolates them.
- Skipping the protector. The first thunderstorm sends a surge down the line into the instrument and the person holding it.
- Building for music. Widening the band does not make speech more recognisable and costs sensitivity.
Safety
Line voltages
Ringing current from a hand magneto is about 60 to 90 V AC (see Magneto Generator), enough to give a painful shock across wet hands. Never crank a magneto while touching the line terminals, and keep the earpiece off your ear when the ringer is connected. A 48 V central battery stays under the 50 V commonly treated as the hazard threshold, but a lead-acid bank can deliver very large short-circuit current; fuse the feed.
What’s Next
- Learn next: Switching Networks, how any two instruments get joined
- Combine with: Long-Distance, what changes when the line runs past a few kilometres
- Foundation: Telephony for building the first working pair